In the mining of oil sand for the extraction of bitumen therefrom, an essentially oil-sand free overburden is first removed and the underlying oil sand ore is mined with bucketwheels. This mining procedure comprises the setting of explosive charges in the ore which charges are detonated to break up the ore and the resulting chunks of oil sand are dug up with large bucketwheels, dropped on a moving belt, and conveyed to the extraction units of the processing plant. In order to set the explosive charges, it is necessary to drill large diameter boreholes of about 6 to about 30 inches, place about one-half of the explosive in the hole, introduce a primer needed to set off the explosive and then complete the filling of the hole with the explosive after which the explosive is set off. For optimum results, i.e., to obtain a maximum cratering effect a spherical charge geometry is used with a charge length to charge diameter ratio of six or less. In such a charge, the energy produced by the expanding gases after detonation is directed radially outward from the center of the charge in all planes passing through the center, and it moves with a uniform, spherically diverging motion. As long as the ratio of the diameter of the borehole to the charge length of the borehole is 1 to 6, the breakage mechanism and the results are practically the same as with a true spherical charge.
In carrying out such procedures, however, two major problems are encountered. One problem is due in part to the moisture at the perimeter of the borehole due to the percolation of water along the outside of the powder column by methane gas. Thus, when or after the primer is introduced and the borehole filled with powder, the primer often becomes postioned adjacent to the borehole wall in the wet, insensitive zone. When this happens, the hole does not shoot thus effecting a "misfire." When this occurs, the holes must be dug out with a backhoe and frequently the holes which misfired are unknown and adversely affect the bucketwheel excavators when they dig the unexploded material.
A second problem with the conventional method of preparing the blasthole for detonation is due to the static charges which build up and can cause a premature, unintended explosion of the methane gas. It is obvious that these problems create hazards and undue costs in the mining operation.